1. Field of the Invention
The present invention relates generally to wireless telecommunications systems, and more particularly to a timing acquisition algorithm for an adaptive antenna array to increase range of signals received from multiple antennas and suppress interference(s).
2. Description of the Prior Art
With the correct symbol timing and carrier frequency, an adaptive antenna array can generate weights to combine signals received from multiple antennas to increase range and suppress interference(s). However, prior to adaptive array combining, the received desired signals may be severely masked by noise and interference(s). To fully make use of the adaptive antenna array, a timing acquisition technique or algorithm is needed.
The objective of timing acquisition algorithms is to locate the correct timing position of a sync word embedded in received signals. One prior art timing acquisition algorithm uses the cross-correlation approach to identify and locate the timing position of the sync word. For each antenna, a correlation of received samples and the designated sync word is performed over a window of samples (9.5 symbol period), and the set of samples that produce the maximum cross-correlation value will be considered as the synchronization samples. However, in the interference-dominated environment, it can easily happen the signal is in deep fade on one of the diversity branches, but the interference(s) are not in a deep fade on that branch. The signal can be severely masked by the interference(s) on that branch and will cause incorrectly chosen of the sync-position on that branch.
In another prior art timing acquisition algorithm, the timing position of the sync word is identified and located using a mean-square-error (MSE) method. First, the antenna with the strongest signal energy is selected. Then, a MSE between the received samples on the selected antenna and the designated sync word is calculated over a window of samples. The set of samples that produce the minimum MSE will be considered as the synchronized samples. However, in the interference-dominated environment, the antenna that has the strongest received signal energy may not be the antenna that has the strongest desired signal energy. There is a significant chance that most of the energy is from the interference(s).
Neither the correlation approach nor the MSE approach use any weighted and combined signals from all the branches which can effectively combat fading and interference. Even though these techniques may work well for two-branch-antenna-diversity system and an low interference environment, these techniques are not suitable for a four-branch-antenna-diversity system.
Another prior art diversity combining technique is designed to combine signals from all the branches using the Maximum Ratio Combining (MRC) technique. Correlation or MSE approaches can finally be used on the combined signals to identify the position of the sync word. This diversity combining technique has been shown to be efficient under flat fading and noise limited environments, but not too efficient under an interference dominated environment.
Another prior art timing acquisition algorithm, called an interference-cancellation-first algorithm, is proposed by Cupo et al. in xe2x80x9cA Four-Element Adaptive Antenna Array for IS-136 PCS Base Station,xe2x80x9d technical memorandum, ATandT Labs and Bell Labs, 1997, and designed to combine signals from all the branches. In this algorithm, diversity combining weights are first generated using a designated sync word and the samples associated with each timing epoch. The received signals in each timing epoch are then weighted and combined. Correlation or MSE approaches can finally be used,on the combined signals to identify the sync position. The combined signal from the sample set with the right sync position should have the highest SIR and will end up with the lowest MSE or highest correlation value at the end.
Although the interference-cancellation-first algorithm is an effective algorithm, since it optimally combines signals from all the branches, the complexity of the technique is high. The combining weights have to be found for samples in each epoch. If the search window size is seven symbols (size of 6.5 symbols is used in DRM and size of 9.5 symbols is used in EDRU), the algorithm has to calculate the covariance matrix, cross-correlation matrix and combining weights 28 times (seven symbols multiplied by four over-sampling values).
Accordingly, a need exists for a non-complex timing acquisition algorithm for an adaptive antenna array which utilizes the signals from all the branches of the antenna array to increase range and suppress interference(s).
The timing acquisition algorithm is a three-step time synchronization technique for locating the sync timing position of a sync word embedded in a signal received at a base station for achieving synchronization between the received signal and the base station within a wireless telecommunications system. The timing acquisition algorithm is preferably processed by a processor located at the base station.
The algorithm gets rid of the unlikely sync timing position for each branch in the first step; gets rid of the unlikely sync timing position for all branches in the second step; and uses optimal diversity combining for the remaining timing position and uses the conventional correlation or mean-square-error (MSE) approach on the combined data in the third step to finally locate the timing position of the sync word. The first two steps limit the computational load of the third step to a reasonable level. For example, if only two sync timing positions remain after the first two steps, then during the third step, weight calculations need only be performed twice, i.e., one for each sync timing position that still remains.